Evaporative Emissions System: Description and Operation

EVAP System

Depending on air pressure and ambient temperature, varying quantities of fuel vapors form above the fuel surface in the fuel tank.

The Evaporative Emission (EVAP) system prevents these Hydrocarbon (HC) emissions from entering the atmosphere.

Fuel vapors pass from the highest point in the tank (on the filler neck) through the fuel tank and into the EVAP canister.

The activated charcoal in the canister stores these gases like a sponge.

When driving with active oxygen sensor control (the engine is warm), the EVAP canister purge regulator valve 1 is activated by the Engine Control Module (ECM) based on load and speed. The opening time is dependent on input signals.

During the cleansing process (regeneration of active charcoal), the intake manifold vacuum sucks fresh air through the breather connection of the EVAP canister. The fuel vapors stored amongst the activated charcoal and fresh air are proportionately supplied to be burned.

Without electricity (for example, an open circuit) the EVAP canister purge regulator valve 1 remains closed. The EVAP canister is not purged.

Leak Detection Function Description

The EVAP system (including, the fuel tank) is equipped with a leak diagnostic that detects leaks in the system.

The diagnosis operates by pressurizing the system and should detect leaks where the damage exceeds 0.5 mm in diameter.

The leak detection pump builds pressure in the EVAP system during the diagnosis. The pump will switch off when the pressure is attained. When the pressure falls to below a certain figure, the pump will switch on again. On Board Diagnostic (OBD) monitors the switch intervals and stores a Diagnostic Trouble Code (DTC) in the DTC memory if the intervals are too short.

• Hose connections are secured with either spring-type or clamp-type clips.

• Use hose clip pliers (VAS 6340) to install spring clips or the hose clip pliers (VAS 6362) are recommended.